There are several methods available for optical resolution of racemic amino acids or racemic amino acid esters. For example, methods known for optical resolution of DL-phenylalanine or an alkyl ester thereof include (1) crystallization into a phenylalanine methylsulfate (see JP-A-6-306029 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")), (2) use of a resolving reagent, such as an N-acylamino acid (see JP-B-51-17522 (the term "JP-B" as used herein means an "examined Japanese patent publication"), U.S. Pat. No. 3,941,831) or a dipeptide derivative (see JP-B-62-56857), (3) use of an enzyme, such as an acylase for resolution of an N-acylamino acid (see U.S. Pat. No. 3,907,638) or a protease for resolution of an amino acid ester (see U.S. Pat. Nos. 3,813,317 and 3,878,043), and (4) acid decomposition of an adduct of a dipeptide derivative and a D-amino acid ester (see JP-B-59-43159).
Each of these methods produces an optical isomer having a relatively high optical purity through relatively easy operations. However, it has been difficult to recover an optically active amino acid or amino acid derivative having a high optical purity of 99% or higher.
In particular, the above-described method for producing an optically active amino acid ester is disadvantageous in that the workability is poor due to poor filterability of the adduct; when the adduct contains impurities, such as an L-amino acid ester and a DL-amino acid, the purification efficiency is poor; and the operation of filtration is tedious for industrial production and requires expensive equipment.
JP-B-2-12238 and JP-B-2-12240 disclose a method in which an adduct of a dipeptide derivative (which is enzymatically produced from an N-substituted-.alpha.-amino acid and a DL-amino acid ester) and a D-amino acid ester is recovered as a slurry in a water-immiscible organic solvent, and the adduct is decomposed with an acid to recover a D-amino acid ester. According to this method, however, the unreacted DL-amino acid ester and a DL-amino acid as a decomposition product are simultaneously recovered, making it impossible to recover a D-amino acid or derivative thereof at an optical purity of 99% or higher.
In these days, optically active amino acids or derivatives thereof that have been increasing in importance for use as raw materials of medicines are required to have a high optical purity as well as a high chemical purity from the standpoint of efficacy and side effects. It has been keenly demanded therefore to develop a method for producing an optically active amino acid or derivative thereof having a high optical purity through simple and easy operations with high industrial productivity.